The invention relates to methods for producing synthesis gas of a desired or adjustable molar ratio of hydrogen to carbon monoxide. The invention further relates to devices which are particularly suitable for carrying out the methods according to the invention.
The term “synthesis gas” or “syngas” denotes gas mixtures that consist mainly of hydrogen and carbon monoxide, but admixtures of carbon dioxide, nitrogen, noble gases, methane and other hydrocarbons may also be present. Synthesis gas is used as a an intermediate product or starting material for many synthesizing processes; for example for the production of methanol and other alcohols, dimethyl ether, synthetic fuels (Fischer-Tropsch synthesis), synthetic natural gas (SNG), ammonia (Haber-Bosch process), and for oxo syntheses. The base materials thus obtainable are important starting materials or intermediates for the chemical industry, for example for the production of active pharmaceutical ingredients, plant protection agents and plastics.
Synthesis gas is usually obtained by catalytic conversion of carbon-containing or hydrocarbon-containing raw materials (such as coal, natural gas, methane). Here, in particular the following chemical reactions are important:CnH2n+2+n.H2O→n.CO+(2n+1).H2 (steam reforming);  (I)CnH2n+2+n.CO2→2n.CO+(n+1).H2 (dry reforming);  (II)CO+H2O→CO2+H2 (water-gas shift reaction).  (III)
Reactions (I) and (II) are endothermic; the technical implementation of this process usually takes place at elevated temperatures (about 700-850° C.), using catalysts such as nickel oxide catalysts or mixed metal oxides.
The synthesis gas thus obtained contains CO and H2 in a specific molar ratio (“molar ratio CO/H2”) which depends on the type and—possibly varying—composition of the raw materials used, and on the method employed.
For example, a method according to the above equation (I) yields a high-hydrogen synthesis gas having a molar ratio of CO/H2 of 1:3 when using methane as a starting material; in the case of ethane, a synthesis gas is obtained having a molar ratio CO/H2 of 2:5, etc.
Accordingly, a method according to the above equation (II) provides a low-hydrogen synthesis gas having a molar ratio of CO/H2 of 1:1 when using methane as a starting material; in the case of ethane, a synthesis gas is obtained having a molar ratio of CO/H2 of 4:3, etc.
For use as a starting material in subsequent synthesis processes, it is necessary or desirable that the molar ratio CO/H2 of the synthesis gas has a certain value or that it lies within a certain range, and that the value or range is maintained as constant as possible, even in the case of fluctuating composition of the starting material.
For example, in the case of synthesis gas to be used for the production of dimethyl ether, a mole ratio CO/H2 of 1:1 is necessary or desirable; for the production of fuels (Fischer-Tropsch process) or of methanol, a CO/H2 ratio of 1:2 is necessary or desirable; and for oxo synthesis, a ratio ranging from 2:3 to 2:5 is necessary or desirable.
In order to produce synthesis gas with a desired CO/H2 ratio, methods for cryogenic separation of raw synthesis gas into product streams that are high in hydrogen and high in carbon monoxide have been proposed; see European patent application publication EP 0 898 136 A2, inter alia. These methods are disadvantageous in several respects, especially as multiple cooling devices, as well as corresponding devices for separating liquid and vapor phases are required for the cryogenic separation.
It is also possible to reduce the CO content in the synthesis gas by water-gas shift reaction (III); however, this has the disadvantage that CO2 is formed.
Another known possibility of adjusting the CO/H2 ratio is the addition of hydrogen, which must, however, be generated by other, energy-consuming methods (such as electrolysis); see German published patent application DE 10 2010 027 474 A1.
A further problem in the production of synthesis gas is that the available raw gases (feed gases) may contain large amounts of impurities or admixtures of other gases (such as H2S). This is true, for example, for feed gases that are generated in biogas installations or pyrolysis plants (biogas, pyrolysis gas). Since these impurities often act as catalyst poisons for the catalysts used in the production of synthesis gas, an expensive pre-treatment for purification of the crude gases is frequently necessary in order to separate the impurities (e.g. desulfurization).